This invention relates to formulations and methods for controlling and suppressing plant pathogens.
Plants including many commercially valuable cultivars ranging from fruit trees and crop plants to ornamental shrubs are affected by pathogens including a wide spectrum of bacterial and fungal pathogens. Taken together bacterial and fungal infestations have a huge economic impact on commercial agriculture. Many pathogenic microorganisms are spread by insect activity consequently much damage attributable to insect infestations is in fact caused by microorganisms which insects spread from infected to uninfected plants. Another common pathway for the spread of infection is wind or rain. Accordingly, a great deal of time and expense is devoted to trying to protect plants from these pathogens or to minimize the damage they do once they infect a given plant.
Bacterial pathogens which do significant damage to cultivated plants, include, for example, Xanthomonas campestric, which causes leaf blight, and X. campestric (which causes Walnut blight). One particularly devastating bacterial pathogen is fire blight caused by the bacterium Erwinia amylovora this bacteria is related microorganisms which cause soft rot diseases such as E. carotovora and E. chrysanthemi, and species of E. Pantoea such as stewartii which causes Stewarts wilt in corn and hervicola. Fire blight infection is characterized by wilting and tissue necrosis. Fire blight itself affects many varieties of commercially important pome fruit trees, many varieties of apple and pear trees are particularly susceptible to fire blight. Other susceptible species include various varieties of stone fruit trees and some ornamental plants.
Affected areas of plants with fire blight appear scorched and blackened, symptoms which give fire blight its common name. Symptoms vary with the susceptibility of the plant and environmental conditions. Effects range from the destruction of specific plant structures to the death of the entire plant. For a more detailed discussion of fire blight the reader is directed to an article by J. A. Eastgate, Molecular Plant Pathology (2000) 1 (6), 325-329 and references therein.
The microorganism which causes fire blight generally enters a susceptible plant through one of five primary routes for infestation these include: formation of a canker; through a blossom; through new root or shoot tissue; or in response to trauma damage caused by storms or by human or animal activity. One common pathway to infection is through over-wintering of the organism in the bark of trees or in a canker on or in the bark. Cankers can be small and are easily overlooked during the winter pruning efforts. During the spring, the pathogen may ooze from the canker in a sticky, sap-like liquid which is readily dispersed by rain, wind, and pollinating insects such as bees. Once dispersed the pathogen may infect blossoms of the same or neighboring plants. Infection of the blossoms is commonly referred to as blossom blight.
Blossom blight represents one particularly devastating form of the infection. Once the first infected stamen appears, pollinators, wind and rain can rapidly carry the pathogen from one bloom to another. An entire orchard can be colonized within a several hours or few days; when environmental conditions are suitable, the pathogen population can double within 20 to 30 minutes. Consequently, fire blight has been known to spread exponentially through stands of susceptible plants. When conditions favor the pathogen it may sweep across an entire apple or pear tree orchard in a matter of only two to three days.
Once fire blight has infected a portion of a plant, growers must act aggressively to isolate the infected portion or in some cases the entire plant. Failure to take the appropriate remedial action immediately may result in loss of the entire orchard or surrounding nurseries. The most common approach to trying to control an outbreak of fire blight is to aggressively prune and in some cases completely destroy the infected plants. It is also common practice to destroy nearby non-infected plants in order to reduce the risk that the infection may spread into the entire orchard.
Additional approaches to fire blight control include treating plants with copper salts and antibiotics such as streptomycin and/or tetracycline. Since treatment after infestation is not always effective, nurseries often resort to prophylactically treating entire orchards with antibiotics to try and reduce the susceptibility of their crops to fire blight. As a result there have been reports of increased amounts of antibiotic residues in fruits, insects, and the soil around some orchards treated for fire blight. Furthermore, the widespread use of antibiotics has helped to select for variants of E. amylovara that are resistance to the antibiotics commonly used against E. amylovora. Despite these efforts, epidemics of fire blight appear to explode in orchards, many of which have no known history of infestation with fire blight. Clearly then, currently used methodologies for the control of fire blight are not particularly effective.
Equally devastating to various commercially important crops are infestations caused by fungal plant pathogens. Fungal pathogens of considerable economic impact include, but are in no ways limited to: Venturia inaequalis, which causes Apple Scab; Gnomonia setecea which causes Birch Blight; Mycosphaerella populorum, which causes Septoria (Leaf Spot) in Dogwood trees; Gymnosporangium globosum, which affects Hawthorn trees and shrubs; and Kabatina juniperii, which cause a from of Juniper Blight. Because the fungal infestations are difficult to treat, growers often take prophylactic action to reduce the risk for infection. Prophylactic activities include pruning, isolating and oftentimes destroying trees or other plants infected with fungal pathogens. Other approaches currently available include the application of chemical and biological fungicides. These approaches are often expensive and they are not always wholly effective in suppressing the infestation or controlling the economic damage done by fungal plant pathogens.
There is a need then for effective, economical formulations and methods for the control and suppression of plant pathogens including those caused by pathogenic bacteria and fungi.